The invention relates to a dielectric resonator antenna (DRA).
The invention further relates to a transmitter, a receiver and a mobile radiotelephone that includes a dielectric resonator antenna.
Dielectric resonator antennas (DRAs) are known as miniaturized antennas of ceramics or another dielectric medium for microwave frequencies. A dielectric resonator whose dielectric medium, which has a relative permittivity of ∈r greater than  greater than 1, is surrounded by air, has a discrete spectrum of eigenfrequencies and eigenmodes due to the electromagnetic limiting conditions on the boundary surfaces of the dielectric medium. These conditions are defined by the special solution of the electromagnetic equations for the dielectric medium with the given limiting conditions on the boundary surfaces. Contrary to a resonator, which has a very high quality when radiation losses are avoided, the radiation of power is the main item in a resonator antenna. Since no conducting structures are used as a radiating element, the skin effect cannot be detrimental. Therefore, such antennas have low-ohmic losses at high frequencies. When materials are used that have a high relative permittivity, a compact, miniaturized structure may be achieved since the dimensions may be reduced for a preselected eigenfrequency (transmission and reception frequency) by increasing ∈r. The dimensions of a DRA of a given frequency are substantially inversely proportional to ∈r. An increase of ∈r by a factor of xcex1 thus causes a reduction of all the dimensions by the factor xcex1 and thus of the volume by a factor of xcex13/2, while the resonant frequency is kept the same. Furthermore, a material for a DRA is to be suitable for use at high frequencies, have small dielectric losses and temperature stability. This strongly limits the materials that can be used. Suitable materials have ∈r values of typically a maximum of 120. Besides this limitation of the possibility of miniaturization, the radiation properties of a DRA degrade with a rising ∈r.
Such a DR antenna 1 in the basic form considered by way of example is represented in FIG. 1. Not only the form of a cuboid, but also other forms are possible such as, for example, cylindrical or spherical geometries. Dielectric resonator antennas are resonant modules that work only in a narrow band around one of their resonant frequencies (eigenfrequencies). The problem of the miniaturization of an antenna is equivalent to the fact of lowering the operating frequency with given antenna dimensions. Therefore, the lowest resonance (TEz111) mode is used. This mode has planes of symmetry in its electromagnetic fields, of which one plane of symmetry of the electric field is referenced plane of symmetry 2. When the antenna is halved in the plane of symmetry 2 and an electrically conducting surface 3 is deposited (for example, a metal coating), the resonant frequency continues to be equal to the resonant frequency of an antenna with the original dimensions. In this manner, a structure is obtained in which the same mode is formed with the same frequency. This is represented in FIG. 2. A further miniaturization can be achieved with this antenna by means of a dielectric medium that has a high relative permittivity ∈r. Preferably, a material that has low dielectric losses is selected.
Such a dielectric resonator antenna is described in the article xe2x80x9cDielectric Resonator Antennasxe2x80x94A review and general design relations for resonant frequency and bandwidthxe2x80x9d, Rajesh K. Mongia and Prakash Barthia, Intern. Journal of Microwave and Millimeter-Wave Computer-aided Engineering, vol. 4, no. 3, 1994, pp. 230-247. The article gives an overview of the modes and the radiation characteristics for various shapes, such as cylindrical, spherical and rectangular DRAs. For different shapes, the possible modes and planes of symmetry are shown (see FIGS. 4, 5, 6 and p. 240, left column, lines 1-21). Particularly a cuboidal dielectric resonator antenna is described in the FIG. 9 and the associated description. By means of a metal surface in the x-z plane, with y=0, or in the y-z plane, with x=0, the original structure may be halved, without modifying the field configuration or other resonance characteristics for the TEz111-mode (p. 244, right column, lines 1-7). The DRA is excited via a microwave lead in that it is inserted into the stray field in the neighborhood of a microwave line (for example, a microstrip line or the end of a coaxial line).
Since there are two planes of symmetry at right angles to each other, the possibilities of miniaturization are limited. In this manner, the volume of a DRA may be reduced by the factor of 4 while the frequency remains the same.
Therefore, it is an object of the invention to provide a dielectric resonator antenna that offers better possibilities of reducing the dimensions.
This object is achieved in that an electrically conducting coating is provided on at least one curved surface into which the tangential component of an electric field of an eigenmode assigned to the dielectric resonator antenna disappears. The antenna may be spherical, cuboidal or have another geometric form that is selected, for example, while taking into account manufacturing or aesthetic conditions. Depending on the shape and the dimensions of the dielectric resonator, the antenna has a discrete spectrum of eigenmodes and eigenfrequencies that may be propagated, which are determined by solving the Maxwell equations for electromagnetic fields with the given boundary conditions. Therefore, defined eigenmodes are always assigned to a given DR antenna. When considering the lowest mode (TEz111-mode corresponds to the least resonance), the smallest dimensions are found for the DRA. Certain subdivisions of the associated electric field inside the antenna are found for the eigenmodes, the field vector of which electric field can be subdivided into a tangential and normal component at any place. According to the invention, such curved surfaces are provided with an electrically conductive coating, which surfaces are featured by a disappearing tangential component of the electric field. This means that on these curved surfaces of the dielectric resonator antenna the same boundary conditions hold as found in an ideal electric conductor. The conducting coating retains these requirements for the electric field and thus also for the assigned eigenmode. The electrically conducting coating on the curved surface is preferably obtained by cutting the DRA along the curved surfaces and covering the intersecting surface with a metal coating (for example, a silver paste). As a result, the volume of the DRA may be reduced considerably, although for the rest the same mode is developed with the same frequency. Since there are a plurality of curved surfaces so featured, a highly advantageous surface may be selected, for example, in dependence on the desired degree of miniaturization, required bandwidth of the evolving antenna and manufacturing conditions.
In a further embodiment of the invention, a cuboid of a dielectric material having the side lengths a, b and d in the orthogonal directions x, y and z is provided for forming the dielectric resonator antenna, and a curved surface having the form {(x,y(x),z), x∈[0,a/2], z∈[0,d]} with y(x)=b/xcfx80 arcsin{C[sin(x xcfx80/a)]a2/b2} covered by the electrically conducting coating. A cuboid is one of the basic forms used for dielectric resonator antennas. This basic form can very well be described by means of a Cartesian co-ordinate system whose zero is advantageously chosen to be in a corner of the cuboid so that the edges of the cuboid lie on the x, y and z axes and positive side lengths a, b and c evolve. Then the curved surfaces may be indicated on the above formula in a very simple manner. The function x(x) then holds for curves in a plane z=const.∈[0,d], so that curved surfaces evolve that are perpendicular to such a cross-sectional plane. Since there are many such curved surfaces, the formula contains a parameter C that may assume any positive value (C greater than 0).
In an advantageous further embodiment of the invention, such a surface formed by means of a parameter C less than 1 is provided for forming the curved surface. Advantageous to the invention is the use of a curved surface that is described by means of a parameter of C less than 1, because then the object of the reduction of the dimensions of the dielectric resonator antenna is achieved very well. This achieves a considerably larger reduction of the volume of the dielectric resonator antenna than is possible without an electrically conducting coating on a curved surface.
The object of the invention is furthermore achieved by a transmitter, a receiver and a mobile radiotelephone having such a dielectric resonator antenna, inside which antenna an electrically conducting coating is provided on at least one curved surface, in which surface the tangential component of an electric field of an eigenmode assigned to the dielectric resonator antenna disappears.